Reversible phase transformations between Pb nanocrystals and a viscous liquid-like phase

Phase transformations have been a prominent topic of study for both fundamental and applied science. Solid-liquid reaction–induced phase transformations can be hard to characterize, and the transformation mechanisms are often not fully understood. Here, we report reversible phase transformations between a metal (Pb) nanocrystal and a viscous liquid-like phase unveiled by in situ liquid cell transmission electron microscopy. The reversible phase transformations are obtained by modulating the electron current density (between 1000 and 3000 electrons Å−2 s−1). The metal-organic viscous liquid-like phase exhibits short-range ordering with a preferred Pb-Pb distance of 0.5 nm. Assisted by density functional theory and molecular dynamics calculations, we show that the viscous liquid-like phase results from the reactions of Pb with the CH3O fragments from the triethylene glycol solution under electron beam irradiation. Such reversible phase transformations may find broad implementations.


Movie S2.
Rotation of Pb core-shell nanoparticles.

Movie S7.
Reversible transformation of lead core-shell nanocrystals with a smaller diameter, 5 nm, between a viscous-liquid-like phase and a jelly phase under a high-dose-rate electron beam (3000 electrons/Å 2 /s) or low-dose electron beam (1000 electrons/Å 2 /s), respectively.

Movie S8.
Reversible transformation of multiple lead core-shell nanocrystals between a solid phase and a viscous-liquid-like phase under random electron beam manipulations (high-dose-rate electron beam, 3000 electrons/Å 2 /s, or low-dose electron beam, 1000 electrons/Å 2 /s), respectively.

Fig. S1 .
Fig. S1.Schematic illustration cut-away view of a regular liquid cell for transmission electron microscope (TEM).

Fig. S2 .
Fig. S2.In-situ synthesis of Pb nanocrystals in the liquid cell TEM.TEM image series of the formation of Pb nanocrystals (movie S1).The black and pink arrows mark two particles.

Fig. S3 .
Fig. S3.Characterizations of lead core-shell nanoparticles.(A) HRTEM, the facet of the lead nanoparticle is (111).(B) EDS spectrum of the particles.The Si peak should come from the liquid cell.

Fig. S4 .
Fig. S4.Sequential TEM images showing the rotation of Pb core-shell nanoparticles.Different colored boxes highlight 4 different nanoparticles.It shows that the shell is liquid-like with changes in configurations (movie S2).The core of each nanoparticle maintains the same size, with the contrast changes indicating the nanoparticle rotation.The scale bar is 20 nm.

Fig. S7 .
Fig. S7.The reversible transformation of Pb nanoparticles.Sequential TEM images of the reversible transformation of Pb nanoparticles.The Pb nanoparticles transformed to viscous-liquidlike phases under 3000 electrons/Å 2 /s.The viscous-liquid-like phases transformed to Pb nanoparticles under 1000 electrons/Å 2 /s.The five cycles are corresponding to the experiment in Figure 2A and movie S3.

Fig. S8 .
Fig. S8.Structure determination of Pb nanoparticles with the crystalline solid and viscousliquid-like phases.(A)Sequential TEM images of the reversible transformation of Pb nanoparticles.(B)Determination of the lattice spacing of the Pb particle over the reversible transformations.The particle size was estimated by fitting the intensity histogram of each image (in real space) and determining the mean value and standard deviation of the background liquid signal.Intensity values lower than ~3 standard deviations of the mean background intensity were classified as belonging to the particle(s), and the total area of the particle region as a function of time is reported in Figure 4D and Figure S8B.

Fig. S9 .
Fig. S9.Reversible Pb nanocrystal transformations under random electron beam manipulations.(A) Sequential TEM images of lead nanoparticles with reaction time for different cycles.(B) The particle sizes of the core (magenta) and shell (green) of lead nanoparticles, were measured from movie S4.(C) The measurement methods for Figure 4B and Figure S9B (movie S4).

Fig. S10 .
Fig. S10.Sequential TEM images and their corresponding FFT images of Pb nanoparticle transformation from crystal to jelly phase under 3500 electrons/Å 2 /s, and then from jelly phase to nanoparticles under 2000 electrons/Å 2 /s.(movie S5) Compared with the nucleation and growth under 1000 electrons/Å 2 /s, the nucleation and growth under 2000 electrons/Å 2 /s are quite different.There are many nuclei and the growth rate is much slower.

Fig. S12 .
Fig. S12.Sequential TEM images and their corresponding FFT images of the reversible transformation of Pb core-shell nanoparticles with a smaller diameter, 5 nm.(movie S7)

Fig. S14 .
Fig. S14.Sequential TEM images with highlighted liquid edge in the cell during the transformation.(movie S8) The liquid edge was marked by a green dashed line.

Fig. S15 .
Fig. S15.TEM images of bismuth core-shell nanoparticles.(A) Low magnification TEM image.(B) Sequential images of the Bi core-shell nanoparticle transformation from crystal to jelly phase.

Fig. S16 .
Fig. S16.The sequential TEM images of Bismuth core-shell nanoparticle transformation for different particles in selected area (A) and selected area (B).

Fig. S17 .
Fig. S17.High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image in liquid cell and corresponding carbon K-edge EELS marked on STEM image.The amorphous (I) area of the Pb nanoparticle and the liquid (II) area.There is a wide peak in the I curve, which may be caused by the fragments of TEG.Oxygen is also found in the shells of nanoparticles.

Fig. S18 .
Fig. S18.The electron energy loss spectroscopy (EELS) spectra of carbon K-edge for triethylene glycol (TEG) under beam with time.(A) With background.(B) After background removal.The intensity of the wide peak around 320 eV increases with time, which may result from the increased TEG fragments (CHO3, C2H4O, etc.).

Fig. S19 .
Fig. S19.The calculation of the energy level of the Pb particles in different states.The energy of an isolated Pb(CH3O)2 is set to zero.The energy states are the distance of the Pb crystal with 120%, 140%, 160%, 180%, and 200%.